Intervertebral implant

ABSTRACT

An intervertebral implant consisting of a disc-shaped spacer made of rigid material and insertable between opposed and adjacent vertebrae in which the opposing sides (1, 2) of the implant bordering the vertebrae are circular discs and have a central raised dome (3 or 4) and roof-shaped projections thereon (5 or 6).

SUMMARY OF THE INVENTION

The invention relates to a intervertebral implant comprising a disc madeof rigid material and having two opposing sides bordering respectivelyadjacent vertebrae. Each of the opposing sides includes a circularfrontal area and a raised dome at the central portion thereof. Theopposing sides further include roof shaped projections surrounding theraised dome, each of these projections having a pair of end faces, and aridge edge and a pair of base edges extending between the end faces. Thebase edges and the ridge edge of these projections form respective areasof concentric circles.

German Offenlegungsschrift (published patent application) 28 04 936 andGerman Offenlegungsschrift (published patent application) 22 63 842, forinstance, describe such a known endoprosthesis.

A drawback with such known endoprostheses is that in order to anchorthem firmly to the vertebra either a complicated endoprosthesis shapeand a corresponding milling of the bone or the use of bone cement isnecessary.

The achievable protection against rotation or sideward dislocation isgenerally problematic due to the large mechanical strain exerted on thespine.

It is therefore an object of the invention to provide an endoprothesisof the intervertebral disc which provides a good and long-lasting fitbetween the vertebrae and with which micromovements and in particularrotations and sideward dislocation can be prevented.

The invention is based on the realization that the sideward dislocationof an intervertebral implant can be reliably prevented by a number ofgeometrical shaping measures whereby this shaping, in the case of around and essentially disc-shaped implant, is such that the stopsdesigned to prevent any movement in a radial and tangential directionare in the form of areas which are essentially oriented in a directiontransverse to these radial and tangential directions of movement. Thecorresponding shaping is in addition such that the parts incorporated inthe areas can penetrate the neighbouring vertebrae and are thereforeshaped as roof-shaped projections. This penetration increases with theamount of loading applied so that the stopping effect also increases.The surface is also such that it can grow into the vertebrae which havebeen milled accordingly. The sideward dislocation or rotation of theintervertebral implant must be prevented by the geometrical shaping forat least as long as the implant has started to grow onto the bone.

According to the invention the frontal areas of the endoprosthesisbordering the vertebrae are circular and each comprise a central raiseddome and roof-shaped projections whose longitudinal base edges formconcentric parts of arcs of circles. The raised dome enables theendoprosthesis to be centered relative to the vertebrae and also helpsto stabilize it with regard to sideward dislocations. However,dislocations and rotational movements are mainly prevented from occuringby the projections.

In an advantageous embodiment of the invention both the size and theshape of the raised dome are formed in accordance with the anatomicalconditions of the vertebrae, in order that the raised dome can then befirmly pressed into the inner softer tissue of the vertebrae. The raiseddome is then preferably convex and spherical in shape.

The roof-shaped projections can be pressed into the spongiosa of thevertebrae so that only one level cut must be carried out on thevertebra. The ridges of the roof-shaped projections preferably form, asdo the longitudinal base edges, concentric parts of arcs of circleswhereby their length is smaller than the length of the longitudinal baseedges. Due to this and due to the centered position of the ridgerelative to the projection area the gable faces are shaped as sloped,upwardly projecting triangles, which not only facilitates the pressingin of the implant into the bone but also facilitates the growing of theimplant onto the bone.

The arrangement of the projections is preferably symmetrical so that thefrontal area seems to consist of a number of similar circle segments.The spacing between each of the segments, the spacing from the edges ofthe segments and the radial surface area of the projections areidentical. The radial length of the longitudinal base edges of theprojections increases proportionally with increasing radial distance. Ithas been shown that four projections arranged adjacent to each other inthe radial direction in each segment and 12 segments is sufficient foranchorage and for ensuring that the conditions are good for growth ontothe bone.

Particularly, a spacer made of rigid material, which cannot deform invertebral movements and whose areas bordering the vertebrae aretherefore greatly strained, can be safely secured in this manner.

The frontal areas of the endoprosthesis shaped as described above andwhich are to be pressed into the milled level bone surfaces arepreferably coated with a porous layer of biocompatible material. Such alayer preferably comprises a so-called madreporation made from amaterial which is identical with the core material or a resorbable layerof polylactide or an equally suitable material--which--when applied to aporous surface--after the implantation--is replaced by newly formed bonematerial.

A circumferential groove running around the lateral surface area of thespacer simplifies the surgical operation. The implant can be held by andinserted using surgical forceps. Spongiosa screws, bone cement or otheranchoring means are no longer required.

If, in accordance with a preferred embodiment of the invention, thefrontal area has an inclination which is symmetrical relative to themiddle plane and preferably of an angle between 3° or 4°, the shape ofthe implant corresponds more closely to the curvature of the spine inthe neck region and enables insofar an optimal fit and load transmissionbetween vertebra and implant.

In particular, a rounded recess is provided, which extends from the edgetowards the centre as far as a depth of essentially 10% of the diameter.This recess forms an passage for the spinal nerve and is preferably inthat part of the circumference towards which the frontal areas converge.

In order to simplify the manipulation of the implant a bore is providedwhich extends from the edge of the implant towards the centre and isprovided with an internal thread over at least a part of its length. Asetting instrument which has a rod or shaft with a correspondingexternal thread is provided to engage with this bore. Furthermore theinstrument is provided with a displaceable casing which comprises atleast one nose-shaped projection at its end facing the external threadof the shaft, the projection engaging with the recess adjacent to thebore of the implant and the recess being part of a circumferentialgroove of the implant. Due to a stop or a corresponding catch thescrewing of the threaded part of the shaft into the bore pulls thenose-shaped projection into the recess and engages with it.

Further advantageous features of the invention are featured in thedependent claims and will be described in greater detail below, togetherwith a description of the preferred embodiment of the invention as shownin the drawings. They show:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a longitudinal sectional view of an embodiment of theendoprosthesis according to the present invention.

FIG. 1a is an enlarged longitudinal section view of detail I of theendoprosthesis shown in FIG. 1.

FIG. 1b is an enlarged longitudinal sectional view of detail II of theendoprosthesis shown in FIG. 1.

FIG. 2 is a plan view of the endoprosthesis shown in FIG. 1.

FIG. 3 is a perspective view of the endoprosthesis shown in FIGS. 1 and2.

FIGS. 4a to 4d are further embodiments of the endoprosthesis accordingto the present invention.

FIG. 5a is a cross-sectional view of the setting instrument according tothe present invention.

FIG. 5b is an elevational view of the setting instrument shown in FIG.5a.

DETAILED DESCRIPTION OF THE INVENTION

The longitudinal section of an endorposthesis of an intervertebral discas illustrated in FIG. 1 shows a central raised dome 3 and 4 on each ofits frontal areas 1 and 2 bordering the vertebrae. Each of the raiseddomes 3 and 4 are surrounded by roof-shaped projections of which two, asexamples, have been designated 5 and 6. The sloped, upwardly projectingedges of the projections 5 and 6 are in the form of cutting edges whichfacilitate the pressing of the prosthesis onto the levelly milledvertebrae. Because the inside of the vertebrae consist of soft tissuethe ridges 6a and 6b of the projections 5 and 6, which extend furtherthan the raised domes 3 and 4, can be pressed into the bone tissue. Inorder to be able to hold and direct the endoprosthesis with a surgicalforcep the lateral surface area of the endoprosthesis has acircumferential groove 7. This groove forms a round depression incross-section and spans approximately half of the height of the lateralsurface area.

FIG. 1a shows an enlarged view of detail Ia of the endoprosthesis ofFIG. 1. The surface of the frontal area 1 is coated with a porousbiocompatible layer 26. This surface coating 26 can either be smoothly26 or roughly structured 26a. The treated and porous surface 27 offrontal area 2 in FIG. 1b is partially filled with resorbable material28 in order to enable the ingrowth of bone material. Such a surfacetreatment 27 and/or surface coating 26 serve to facilitate growth ontothe bone.

FIG. 2 shows a plan view of one of the two similarly shaped frontalareas 1 or 2 of the endoprosthesis. The round frontal area 1 or 2 issubdivided into one central region in which the raised dome 3 or 4 issituated and four ring-shaped regions which surround the raised dome 3or 4. Twelve roof-shaped projections 5 or 6 are situated in each of thering-shaped regions and the shape and the size of the projections 5 or 6are the same in each of the ring-shaped regions. The surface structureof the frontal areas 1 and 2 comprises twelve identical segments. Thesegments, together with the raised dome 3 or 4, prevent an unintentionaldislocation in any radial direction.

The longitudinal base edges 8 and 9 of the ridge 10 of the roof-shapedprojections 5 or 6 form concentric arcs of circles whereby the ridge 10is not as long as the longitudinal base edges 8 and 9. In that way, thegable end faces 11 of the roof-shaped projections 5 or 6 inclineupwardly. The projections are therefore shaped like a hip roof. Anunintentional rotation of the intervertebral implants can be preventedin both tangential directions due to these gable end edges.

The roof-shaped projections 5 or 6 as arcs of circles are longer withincreasing distance from the center. They are symmetrically situated onradii of the round frontal areas and form a ray-like pattern. Regionslying on the same level as the frontal areas of the essentiallycylindrically shaped endoprosthesis remain in between the projections.

A perspective view of the endoprosthesis of the intervetebral disc isillustrated in FIG. 3 as a wire model in order to emphasize the surfacestructure.

A further variation of the intervertebral implant according to theinvention is shown in the FIGS. 4a to d. The elements which correspondto those elements of the previously described embodiment are given thecorresponding numerals onto which an "'" has been added.

In the sectional view of FIG. 4a it can be seen, that both frontal areashave an inclination of approx. 7° relative to each other. The directionof inclination is chosen to be symmetrical to the middle plane for bothfrontal areas and is therefore approx. 35° between the middle plane andeach frontal area. The diameter of the implant is between 16 and 20 mmwhereas the height is between approx. 6 and 9 mm. The total inclinationof the frontal areas is increased with increasing diameter of theembodiments corresponding to different vertebrae sizes so that thegreatest inclination corresponds to the greatest diameter and viceversa.

In the plan view according to FIG. 4b a rounded recess 12 can be seen,which has a depth of 10% of the diameter. The recess is substantiallycylindrical, whereby the axis of the rounding runs parallel to thegeometrical axis of the implant. The recess prevents the nerve tracts inthe spinal region from being blocked by the implant. The radius of therounding is approximately equal to half the radius of the frontal areas.As can also be seen from FIG. 4d the recess is on that side of theimplant with the smallest distance between both frontal areas.

It can be seen from the sectional view according to FIG. 4a and alsofrom the side view according to FIG. 4c that the shown implant has abore 13 which extends beyond the middle region by about half a diameterof the dome 3' or 4'. A metrical internal thread 14a is provided betweenan outer enlarged area 14 and the end of the bore. The outer enlargedarea 14 prevents the bone from being able to grow into the threadedbore. In addition, another cap--not shown in the FIGURE --made from anelastic biocompatible material, for example silicon rubber, is provided,with which the opening can be closed over after the implant has beeninserted.

The bore serves to receive an instrument to manipulate an intervertebralimplant, the instrument being shown in FIG. 5a in the plan view insection and in FIG. 5b in side view.

On a slim shaft region 15 a striking weight 16 is displaceable betweentwo stops 17 and 18 so that an insertion or removal of the implant whichis connected to such a setting instrument can take place in accordancewith the strike direction.

The stop 17 is formed by the front end of a casing 19 which islongitudinally displaceable along the shaft 15 by a set distance. Thecasing 19 has a screw 20 which is firmly connected to the casing 19 andwhose end which faces the middle axis of the casing projects into theinside of the casing. This end engages with a region 21 of the shaft 15having a reduced diameter. In this way, the casing 19 can not be totallyremoved from the shaft 15. The casing is oval in cross-section and hasprojections 22 and 23 on the side opposite the stop 17 which grip intothe corresponding recesses of the implant.

A free threadless shaft end 24 corresponds in its diameter to the bore13 and the adjoining external thread 25 corresponds to the internalthread 14a of the bore.

To insert the implant the free end of the shaft 24 with the externalthread 25 is inserted in the bore of the implant and is firmly screwedin. The noses 22 and 23 are thereby pulled into the circumferentialgroove of the implant and thereby make it possible for the position ofthe implant to be controlled by holding the casing 19. The implant cannow be driven in with the set weight 16. After the final position hasbeen reached the instrument is removed. In order to remove the implantduring reoperation the steps must be carried out in the opposite order.

It will be understood that the above description of the presentinvention is susceptible to various modifications, changes andadaptations, and the same are intended to be comprehended within themeaning and range of equivalents of the appended claims.

We claim:
 1. An intervertebral implant insertable between two vertebrae,comprising:a disc made of rigid material and having two opposing sidesbordering respectively adjacent vertebrae, each said side having acircular frontal area, a raised dome at a central portion of saidcircular area, and roof-shaped projections surrounding said dome, eachof said projections having a pair of end faces, a ridge edge, a pair ofside faces, and a pair of base edges extending between said end faces,each of side faces terminating at said ridge edge and at a respectivebase edge the base edges and the ridge edge of said projections formingrespective arcs of concentric circles.
 2. The intervertebral implantaccording to claim 1, wherein each of said raised domes is convex andspherically shaped, and wherein an outer diameter of each of said raiseddomes corresponds to an inner diameter of a marrow cavity of arespective one of the vertebrae.
 3. The implant according to claim 1,wherein said end faces comprise upwardly sloped projecting triangles theridge edge on each of said projections being shorter in length than thebase edges of the same projection, and each said side having levelregions between said roof-shaped projections.
 4. The intervertebralimplant according to claim 1, wherein the ridge edges of the projectionson each side of said disc are of increasing length in a radially outwarddirection.
 5. The intervertebral implant according to claim 1, andfurther comprising means for promoting bone tissue resorption located onat least portions of said circular frontal areas.
 6. A rigidintervertebral implant according to claim 1, wherein said disc has alateral surface area that is substantially cylindrical, said lateralsurface area including a circumferential groove.
 7. A rigidintervertebral implant according to claim 1, wherein said disc isrotationally symmetrical about a central axis.
 8. The intervertebralimplant according to claim 1, wherein said disc is symmetrical about aplane of symmetry mid-way between said sides.
 9. The intervertebralimplant according to claim 8 wherein each said frontal circular area isdisposed at an angle of between 3 to 4 degrees with respect to saidplane of symmetry.
 10. The intervertebral implant according to claim 1,wherein said disc includes a substantially cylindrical lateral surfacehaving a central axis and a rounded recess in said lateral surface, saidrounded recess having a geometrical axis that is parallel to saidcentral axis.
 11. The intervertebral implant according to claim 10,wherein said disc has a diameter and said rounded recess extendsinwardly toward said central axis to a depth of about 10% of thediameter of said disc.
 12. A rigid intervertebral implant according toclaim 11, wherein the sides of said disc are non-parallel relative toone another and said rounded recess is disposed at a region of saidlateral surface where said sides are closest to one another.
 13. Theintervertebral implant according to claim 1, wherein said disc has asubstantially cylindrical lateral surface defining a central axis and abore extending inwardly from said lateral surface toward said centralaxis.
 14. A rigid intervertebral implant according to claim 13, whereina portion of a surface along said bore includes an internal thread. 15.An arrangement comprising the intervertebral implant according to claim14, and a tool for manipulating said implant, said tool comprising:(a) ashaft having one end with an external thread corresponding to theinternal thread along said bore; and (b) a displaceable shaft casingsurrounding said shaft and having one end adjacent to said shaftexternal thread and provided with at least one projection for engagingwith an outer region of said bore during an insertion of said implantbetween two vertebrae.
 16. An arrangement according to claim 14, whereinsaid shaft further comprises two stops and a weight displaceable betweensaid two stops.